This application relates to hard disc drives and more particularly to an apparatus and method for efficiently storing and retrieving audio-visual data on a hard disc drive.
Traditionally, disc drives have been designed in accordance with certain fundamental assumptions regarding the importance of data integrity and data flow. For example, it has traditionally been assumed that informational integrity is of paramount concern; even a single-bit error in loading an executable file could cause untold run-time ramifications. As a result, a traditional disc drive would attempt to read target data, perhaps multiple times, if the drive determined that a data error had occurred during a read operation. Additionally, conventional disc drive performance has been measured by assessing the average time consumed in performing a command. As a consequence, it is acceptablexe2x80x94even desirablexe2x80x94for a conventional disc drive to attempt to read and re-read data until the data is either read correctly, or until there is a miniscule probability that the data can actually be read properly. The time consumed in multiple re-attempts of a single read operation is of little consequence to average command times (the metric against which conventional disc drives are measured), if this sort of multiple-retry event occurs infrequently.
A disc drive which is optimized for audio-visual data storage and retrieval should be designed according to different fundamental assumptions regarding the importance of data integrity and data flow. Data integrity is of diminished importance for such a disc drive. A minor inaccuracy stemming from a small error would be minimally observable, if at all, by a viewer of the visual signal. However, reliable data flow is critical for an audio-visual drive. For the sake of illustration, consider an individual viewing a video signal. Such a viewer may not be able to notice a small error in a data stream, but the viewer will notice a three-second delay introduced into the data stream by a drive which repetitively attempts to correctly re-read an errant bit. Accordingly, the performance of an audio-visual drive should be measured against the maximum time consumed to complete a command. Worst-case scenarios for command completion, rather than average-case scenarios, are of central concern in an audio-visual disc drive.
The decision to optimize data integrity and minimize average command times in conventional disc drives has influenced many design considerations in such drives. Many of the design choices made to accommodate the requirements of a conventional disc drive are inappropriate for an audio-visual disc drive.
A conventional disc drive is designed to minimize average seek times. To accomplish this, the actuator arm is designed to be light, so as to minimize its rotational inertia. Also, the magnetic force imparted on the actuator arm is maximized by immersing the coils on the windings of the actuator arm in a strong magnetic field. Both of these mechanical choices, taken in concert, maximize the acceleration of the actuator arm, thereby minimizing average seek times. A consequence of those design choices is that the actuator arm approaches an unstable state. As a result of this instability, a small fraction of disc commands may occasionally require many retries to function properly, thereby yielding a very high maximum command completion time (a property undesirable in audio-visual disc drives).
Conventional disc drives have also been designed to operate at relatively high spin rates, so as to minimize rotational latency. An effect of operating at a high spin rate is that track following is made less reliable, meaning that an occasional command may take several retries to be properly effectuated. Once again, since audio-visual disc drives need to minimize command completion times for worst-case scenarios, this effect is undesirable.
Additionally, conventional disc drives have been designed to optimize informational integrity with respect to the manner in which such drives react to read-time and write-time errors. As will be discussed in greater detail, conventional disc drives behave sequentially duing read and write operations. As an example of sequential behavior, a conventional disc drive will not attempt to read a particular sector until the previous sector has been correctly read. If a read-time error occurs with respect to a given sector, a conventional drive will cease further attempts at reading successive sectors, until the disc has completely rotated and the errant sector can be properly read. Only then, will a conventional disc drive attempt to read successive sectors. A conventional disc drive behaves in a parallel manner during write commands, as well. In short, a conventional disc drive preserves data integrity at the expense of data flow. Accordingly, there is a need for a disc drive that responds to certain read-time and write-time errors in a manner that preserves data flow.
The method and apparatus in accordance with the present invention solves the aforementioned problem and other problems by responding to a certain read-time error in a manner that preserves data flow. More specifically, in response to a read-time error in which a data sector is misread, the disc drive responds by transferring the errant data to the host nonetheless, and then by informing the host of the error. The data may be partially corrected by an error correction function.
The aforementioned behavior is a departure from the traditional sequential paradigm of a conventional disc drive. The new behavior preserves data flow, a quality important to audio-visual drives.